Transmit-receive switch tube structure



1960 w. J. STANNEY ETAL 2,949,562

TRANSMIT-RECEIVE. SWITCH TUBE STRUCTURE Filed March 12, 1958 I00 200 300400 500 600 44 HOURS OF LIFE HOURS OF LIFE 4O 5O 6O 7O 8O 90 I00 IIO I20INVENTORS TEMPERATURE DEGREES CENTIGRADE WILLIAM J STANN-EY on? GEORGEW. CALEY ,tssz

Patented Aug. 116, i960 inc 2,949,562 TRANSbrlT-WCEIVE SWITCH TUBETRUCTURE Wiliiam d. Stanney, Needham Heights, and George W.

Caiey, Hamilton, Mass, assignors, by mesne assignments, to SylvaniaElectric Products Inc, Wilmington, Bel, a corporation of Delaware FiledMar. 12, 1953, Ser. No. 720,932

1 Claim. (Cl. 315-39) This invention relates to gaseous dischargeswitching devices or" the transmit-receive type, and more particularlyto an improved structure and method of assembly of such devices.

Transmit-receive (TR) switching devices are employed in radio directionand-ranging systems to permit the use of a common antenna for bothtransmission and reception of radio frequency energy. Duringtransmission of the high power pulse from the transmitter, the devicedecouples the sensitive receiving apparatus by means of ionization of asuitable gas filling under reduced pressure contained within theenvelope of the device. This decoupling action, together with theposition of the TR switch in the system, insures that essentially all ofthe transmitted energy is directed to the antenna. Refiected radiofrequency signals received by the same a11- tenna are at too low a powerlevel to cause ionization of the gas filling and are thus permitted topass through the TR switch to the receiver.

Gaseous discharge switches of the TR type conventionally contain amixture of gases, a necessary component of which is water vapor whichserves as an ele tron capture agent. After cessation of a radiofrequency discharge in the tube, free electrons remaining in thevicinity of the discharge gap cause a loss in intensity of the returnlow level signal. For satisfactory operation of the switch, therefore,it is necessary that these electron-s can be removed from the gap. Watervapor is an effective electron capture agent, forming negative watervapor ions having low mobility which do not extract appreciable energyfrom the returning low level signal. The rate of removal of the freeelectrons, commonly known as recovery time, is a critical characteristicof switches of this type, and is a determining factor in the minimumoperating range and minimum target size of the radar system in which itis employed.

An auxiliary electrode is commonly employed in the TR switch to providea source of electrons to aid in the ionization of the gas mixture withinthe envelope to promote a rapid discharge upon the incidence of a pulsefrom the transmitter. Such electrodes are now referred to in the art asthe keep-alive or ignitor electrode. This electrode is introduced intothe TR switch tube through an axial passage in a conical discharge gapelectrode and is supported by the outer envelope of the tube. A directcurrent sustained glow discharge is maintained in the region extendingfrom the tip area by biasing the ignitor electrode negatively withrespect to the adjacent conical discharge gap electrode. Without thepartial ionization of the discharge gap provided by the ignitorelectrode sufiicient amounts of radio frequency power to damage thereceiving apparatus may be permitted through the TR switch before agaseous discharge is initiated. It is, of course, desirable that theswitch have such characteristics that the voltage required to initiatethe direct current discharge will be constant over all operatingconditions of the tube; for example, with increases in ambienttemperature and life of the tube. The auxiliary direct current voltagesupply for the keep-alive electrode is necessarily designed aroundspecific current limits when incorporated in a radar system with theconsequence that changes in characteristics of the gas in the dischargeregion may result in a sufficient variation in current to cause the glowdischarge to be extinguished. It is common practice to employ the termvoltage drop to define the difference in potential between thekeep-alive electrode and the adjacent discharge gap electrode for aspecified value of keep-alive current. Any increase or decrease in thisvoltage drop caused by changes in the characteristics of the gas in thedischarge region will mean a corresponding inverse variation in currentwith a fixed voltage supply. A decrease in ignitor current could leadto: (a) relaxation oscillations of the discharge, with the consequencethat the gap may not be ionized upon the incidence of a transmittedpulse, a condition which would allow higher leakage power to reach anddamage the sensitive receiver; or (b) an increase in the power leakagecharacteristic of the device, a condition which would allow higherleakage power to reach and damage the sensitive receiver.

In the process of conducting investigations into various solutions tothis problem, we have noted that when the ambient temperature of thetube is elevated, the gap resistance of the tube increases, the ignitorelectrode voltage drop increases, and with a keep-alive voltage supplyof fixed potential, the ignitor current decreases. We have theorizedthat this phenomena is caused by variations in the partial pressure ofwater vapor within the tube with changes in temperature. We haveconcluded that these variations in water vapor pressure result from thetemperature-pressure characteristics of certain flux constituents whichenter the eifective volume of the tube while joining the parts thereoftogether with soft-solder.

During actual operation of the device under high peak power conditions,the partial pressure of water vapor also increases, thereby exposing theignitor electrode to a more corrosive atmosphere than with lower watervapor pressure. Consequent deterioration of the ignitor electrodematerial causes a decrease in the effective area or the electrode,which, in turn, causes an increase in gap resistance and a correspondingdecrease in ignitor current, creating the. possibility of relaxationoscillations and causing increased power leakage with the attendantundesirable properties mentioned earlier. It therefore follows that thepartial pressure of water vapor should be sufficient only to provide asatisfactory recovery time for the tube. Heretofore, in order to achievea satisfactory recovery time in a new tube it has been necessary toemploy a higher water vapor pressure than was required after a period ofoperation, with the obvious consequence of undue electrode deteriorationwith extended use.

In the assembly of a number of TR tube types, certain structuralcharacteristics of the components of the tube require that the gasreservoir sub-assembly be soft-soldered to the body of the tube, thissoft solder serving to hermetically seal the two parts together as wellas providing a mechanical connection. Although this solder connection isquite remote from the discharge region, we believe that constituents ofthe flux make their way into the active volume of the tube at the timeof soldering, where they are deposited as salts. These flux salts beinghygroscopic compounds (zinc chloride and ammonium chloride), absorb someof the water vapor that is added to the basic fill gas of the tube,which is normally hydrogen or argon. Thus, after the tube is sealed oil,the resultant partial pressure of water vapor is affected by and is inpart determined by the presence and amount of hygroscopic salts presentin the tube. Since, as was noted earlier, the recovery timecharacteristic of the switch is determined by the active water vaporwithin the gas volume, the initial recovery time characteristic of thetube is variable according to the water vapor pressure. Moreover,because the flux salts are hygroscopic, water vapor is released andabsorbed according to the vapor pressure versus temperaturecharacteristic of the compounds involved. Tests which we have conductedhave shown that the pressure of water vapor increases with increasedtemperature, resulting in a lower recovery time. From what has been saidearlier it is desirable that the recovery time be substantially constantover all conditions of operation of the switch and preferably as shortas possible, in the range of 1.5 to 2.5 microseconds. It has been found,however, that these characteristics could not be achieved with tubesassembled with the soft soldering method described, recovery times inthe range from 3.0 to 7.0 being about the best that could be achievedwhile still maintaining other electrical parameters constant. Coupledwith an unsatisfactory recovery time, prior art tubes which have beensoft soldered have also failed to meet rigid specifications, for examplethose established by the military, in respect to ignitor current driftwith increases in temperature.

With an appreciation of the foregoing shortcomings of prior art TRdevices, it is a primary object of the present invention to provide atransmit-receive gaseous switching device having stabilized electricalparameters at elevated temperatures.

Another object of the invention is to provide a transmit-receive devicehaving a longer life during which the electrical characteristics of thetube are stable.

Still another object of the invention is to provide a transmit-receivegas switching tube having a low initial recovery time which does notappreciably change during the life of the tube.

These and other objects are attained in accordance with the presentinvention by means of a seal within the tube structure to protect thegaseous filling of the tube against the flux salts incident to the softsolder seal. This seal is made prior to the soft solder seal during theassembly process so as to eliminate the possibility of flux saltsentering "the active region during assembly. Preferably this seal isformed from a material which is sufficiently fluid to be readily appliedin uniform fashion to the area to be sealed. For this purpose resinouscompositions with cure or set as high melting or infusible materials,e.g., resins of the phenolic, polyester, or epoxy type, are particularlyuseful. Epoxy resins have been found eminently satisfactory because oftheir ease of application, their ability to withstand the operatingtemperatures of the tube, and their freedom from deleterious reactionproducts. The resinous material is applied to the area to be sealedprior to assembling the reservoir with the body portion, and upon curingprevents the entry of flux constituents into the active volume of thetube during the subsequent joining of the parts with soft-solder.

Other objects, advantages and features of the invention will be evidentfrom the followin detailed description when read in connection with theaccompanying drawings, in which:

Fig. l is an elevation view, partially in section, of a commerciallyavailable TR tube (1B24A) showing the application of the inventionthereto;

Figs. 2 and 2A are curves showing like characteristics of a tube of thetype shown in Fig. 1 without and with the seal of the invention,respectively; and

Fig. 3 depicts curves showing the elfect of the invention on the ignitorcurrent versus temperature characteristic of the tube of Fig. 1.

Referring now to the drawings, Fig. 1 illustrates a transmit-receivedevice which is available commercially, and is referred to in the art asan integral cavity type. The main body of the tube comprises a pair offlanges, one of which is shown in the drawing at 10, joined to opposite4 sides of a block of metal having a generally rectangular outer surfaceand a cylindrical axial passage therethrough. Spaced within the centralportion of the axial passage, that is, at the section designated 12, andsecured to the cylindrical wall are a pair of opposed conical dischargegap electrodes 14 and 16 having their large ends spaced apart to definetherebetween a resonant cavity 18. The converging ends of electrodes 14and 16 define between their opposing ends a discharge gap 20. Electrode14 is normally of solid construction, whereas electrode 16 is hollow asshown, and has a small opening at its apex, into which the keep-aliveelectrode is inserted. Axial adjustment of the gap spacing to thefrequency of operation may be made by deforming the flexible diaphragmon which electrode 14 is positioned with a suitable differential screwmechanism generally shown at 22 and disposed within the lower portion ofthe axial opening through the tube body. An exhaust tubulation 24extends into the body member in the region of the discharge electrodesto communicate with the resonant cavity 13, and is utilized forevacuation as well as filling with the gas mixture under reducedatmospheric pressure. Discharge electrode 16 is held in place on a stepon the axial bore by a circular member 26, the elements thus fardescribed all being secured together by a hydrogen atmosphere brazingmethod which does not involve the use of flux. The TR switch is insertedin a wave guide system with the flanges 10 disposed perpendicularly tothe longitudinal axis of the wave guide, and energy is transmitted intoand out of the cavity portion 18 by a pair of coupling windows, one ofwhich is shown at 28.

The other major portion of the switch, which is normally assembled as asub-assembly and later joined with the body portion just described, is areservoir for gas, normally argon or hydrogen and water vapor. Thereservoir comprises a glass bulb or envelope 30 to one end of which ametal tube 32 is joined with a suitable glass-to-metal seal. An ignitoror keep-alive electrode 34 is supported at one end by means of aglass-to-metal seal 36 to the upper end of the reservoir or bulb 30.Electrical contact to a direct current voltage supply and externalcircuitry may be made by means of top cap 38 sealed to the outer end ofelectrode 34. After assembly (the steps of which will be describedhereinafter) the inner tip of electrode 34 extends axially within thedevice through a passage 26a in disc 26 and into conical gap electrode16 to a point a critically short distance from the apex of saidelectrode. conventionally the ignitor electrode is insul-atinglysheathed in glass except for its tip diameter.

The upper portion of the main body of the tube is normally provided witha cylindrical bore, designated at 40, of a diameter to receive thecylindrical tubing 32. Heretofore, the tube 32 was inserted into thebore 40 and bottomed on disc 26 and thereafter sealed around theperiphery by soft soldering. Exhaustive attempts have been made tosubstitute silver soldering for the soft solder at this seal withoutsuccess due in large measure to the large quantity of glass in thereservoir bulb 30 and the danger of destroying the glass-to-metal sealbetween cylinder 32 and the glass envelope by the high temperaturesinvolved with silver soldering. Heli-arc welding has also beenattempted, but the costly equipment and additional assembly timeindicated to be necessary for success have made this approachprohibitive. Practical considerations therefore have dictated that theseal at 42 be made with soft solder, and it is the constituents of theflux used in making this seal that find their way into the active regionof the tube to produce the deleterious results outlined above.

In accordance with the present invention, flux constitucuts from thesoft-solder seal are prevented from entering the active volume of thetube by a second seal 44 applied prior to the making of the soft-solderjoint. For ease of application and to insure sealing against the fluxconstituents, resinous compositions which cure as high-melting orinfusible materials are preferred in making the seal. Particularlyuseful for this purpose is a flexible epoxide casting resin known asStycast 2340 M which is available from Emerson & Cuming, Inc., 869Washington Street, Canton, Massachusetts. Styoast 2340M is a two-partcasting resin requiring no catalyst addition, the two components ofwhich are simply mixed and cured to produce an opaque resin which isextremely tough while quite flexible It adheres very well to metals,plastics, and glass.

In the assembly of the TR tube in accordance with the invention, the twocomponents of the resin as received from the manufacturer are mixed toproduce a flowable plastic mass which, for convenience of application,is inserted in a squeeze tube having a long spout. The tip of the spoutis inserted into the bore 40 and bead of the plastic material appliedaround the periphery of disc 26 where it joins the wall of the bore.Thereafter, before the resin has set, cylinder 32 of the reservoirsub-assembly is inserted into the bore and seated on the disc 26, theresin flowing around the lower end of the cylinder 32 to completely sealthe two parts together at this point. To cure the resin, the assembledtube is placed in an oven at a temperature of about 150 F. for a periodof 8 to hours. The resin cures tack-free, is very tough, and has beenfound to provide an hermetic seal between the cylinder 32 and the bodyportion of the tube. However, to improve the ruggedness of the tube, thecylinder is preferably also soft-soldered to the body portion at 42after the resin has set. It will be noted, however, that with thesoft-soldering operation following the making of the resin seal, fluxconstituents which may flow into the space between tube 32 and bore 40during the application of the solder cannot reach the cavity 18 or thegas reservoir 30, 32. The favorable properties of this resin outlinedabove are retained over a temperature range of --40 C. to +150 C. withthe consequence that the sealing properties are retained over the normaloperating temperatures of the tube and therefore protect the cavity 18from flux constituents throughout the life of the tube.

That the provision of the epoxy resin seal is effective to stabilize theelectrical characteristics of a transmitreceive switch will be evidentfrom an examination of Figs. 2 and 2A which are plots of certainelectrical characteristics of a 1B24A tube, without and with the seal,respectively. A longer test was performed on the tube with the seal sothe curves do not give a full direct comparison, but the curves of Fig.2A emphasize that the characteristics are improved over the first 500hours of life and that these characteristics are maintained forconsiderable longer periods. The curves are those conventionally used toindicate TR tube performance and are as follows:

E =iguitor voltage drop P.L.=peak leakage power L =inserti0n loss AL=increase in insertion loss when keep-alive voltage is on The ordinatesof the graphs of Figs. 2 and 2A have not been labeled, the significanceof the numbers being as follows: ignitor voltage drop is measured inhundreds of volts; peak leakage power is measured in tens of milliwatts;insertion loss is measured in db, and the increment in insertion loss,AL,, is measured in tenths of a db.

Considering first the curves of Fig. 2, it will be noted that withoutthe seal the ignitor voltage drop increases in value from about 340volts to approximately 475 volts over the 500 hours of operation. Somevariation is also noted in the peak leakage power and it will be notedthat there is a considerable increase in AL, when an increase in E wasobserved. The recovery time of the unsealed tube was not tested, but ingeneral it had been observed on tests of many tubes manufactured withoutthe seal that the recovery time varied between 3.0 and 7.0 microseconds,and stayed fairly constant throughout the life of the tube.

The improvement afforded by the seal will be readily evident from Fig.2A wherein it is seen that the ignitor voltage drop was substantiallyconstant at approximately 380 volts throughout the 1000 hours of thelife test, the peak leakage power was of the same order of magnitude aswithout the seal but was more constant throughout the life of the tube,and the incremental insertion loss when the keep-alive is energized issubstantially less and essentially constant over the life of the tube.The recovery time of the tube with the seal was measured, and the plotindicates that the initial recovery time was slightly less than 2microseconds and remained at that value for approximately 500 hours oflife at which time it dropped slightly.

The curves of Fig. 3, which show the effect of variations in temperatureon the ignitor current, also clearly illustrate the advantage obtainedwith the seal. In tubes without a seal, the ignitor current dropped invalue from about microamperes to approximately 60 microamperes, over atemperature range from 20 C. to 120 C., whereas in tubes with the sealthe ignitor current was essentially constant over this range. Thisconstancy of current prevents the relaxation oscillator characteristicsof the discharge which often occurs when the ignitor current falls offdue to increase in temperature.

Having discovered and explained the reasons for certain untoward effectsin prior art TR tubes, and having disclosed a specific embodiment ofmeans for eliminating these eifects in a particular tube type, theapplication of the invention to other TR types and other dischargedevices when the flux problem exists will now be apparent to onesskilled in the tube art. Also, having suggested a specific resin formaking the seal and outlining its desirable properties, othersatisfactory sealing materials will be apparent to those skilled in theresin art. Accordingly, the invention need not be limited to thespecific details illustrated and described but should be regarded assubject only to the limitations of the appended claim.

What is claimed is:

A method for assembling a transmit-receive gaseous discharge device of atype having a body portion provided with an internal cavity and a borecommunicating with the cavity, and a gas reservoir including a glassbulb and a metal tube insertable in said bore, comprising the steps of:applying around the bottom of said bore a bead of fluid epoxy castingresin which upon curing forms a high-melting, flexible resinousmaterial, inserting said tube into said bore to engage said bead ofresin, curing the assembly at a suitable temperature and for a period tocure said resin whereby to form an hermetic seal between the inner endof said tube and said bore which does not release harmful constituentsinto said cavity, and thereafter soft soldering said tube to said bodyportion at the outer end of said bore, the seal formed by the resinprecluding the entry into said cavity of harmful constituents of theflux employed in the step of softsoldering.

References Cited in the file of this patent UNITED STATES PATENTS OTHERREFERENCES Modern Plastics, November 1950 (pages to Ethoxylines articleby E. Preiswerk et a1.

